(1) Field of the Invention
The present invention relates to automatic line equalizers, and, in particular, to such automatic line equalizers of a digital type.
(2) Description of the Prior Art
In a digital communication system, a digital transmission signal suffers from distortion and/or loss during transmission through a transmission line, the distortion and/or loss causing inter-symbol interference. In order to compensate such distortion and/or loss, a repeator or a receiver is provided with an automatic line equalizer.
A known automatic line equalizer comprises an automatic line-build-out (ALBO) network, a fixed equalizer coupled to the ALBO, and a peak detecting circuit at an output of the equalizer, as shown in "Local Distribution System" written by Bender et al, THE BELL SYSTEM TECHNICAL JOURNAL, MAY-JUNE 1975, VOL. 54 NO. 5, pages 919-942 (Reference 1). The peak of an equalized signal from the equalizer is detected by the peak detecting circuit, and the ALBO is controlled by the detected peak signal to match the equalizer.
However, the ALBO, the equalizer, and the peak detecting circuit are constituted by analog circuits and are therefore difficult in formation on a single large-scaled integrated circuit (LSI) of modest size.
Another known automatic line equalizer is, so called, a Decision Feedback Equalizer (DFE) which comprises a transversal filter having a plurality of tap gains. The transversal filter delivers sampled pulses which are decided at a decision circuit to produce decided pulses. The characteristic of the transmission line is estimated from the decided pulses at a control circuit to thereby control the plurality of tap gains. Thus, the transversal filter produces an equalized signal as an output of the filter. Reference is made to "Automatic Equalization for Digital Communication" by Lucky, THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1965, VOL. 44 NO. 4, page 547 (Reference 2).
The DFE can be implemented as a digital circuit and therefore can be contained on an LSI. However, since the estimation of the transmission line characteristic is based on the decided signal, equalization cannot be realized when the decided signal is not substantially or approximately equal to a transmission signal sent out to the transmission line.